A general problem with devices for controlling flow volumes using a flow controlling device which is pivotally arranged about an axis within a channel is that they are relatively expensive, since the production thereof is relatively complex. The production very often comprises several steps with different kinds of treatment, such as moulding, grinding, turning, assembly, control of sealing capacity, etc. It is common knowledge that each extra treatment implies an increase in the final price of the product, which is undesirable.
A method for producing a ball valve is disclosed in FR-A-1 327 745 in which a valve body in the shape of a sphere with a through passage is first prefabricated and then placed in a mold. A core is inserted through said passage, which core, together with the mold and valve body, delimits a cavity which corresponds to the valve housing. A plastic material is then injected into said cavity to thereby form said housing. Because the valve body in effect acts as a core during the molding of the housing, the housing needs no further machining to acomodate the valve body. A similar method to that described above is disclosed in FR-A-2 028 256, though in this case a two part core is used. Because molding of the housing occurs with the valve body in its fully open position, the interacting surfaces of the housing and valve body totally correspond to each other only in said fully open position. Since it is normally desirable to have the best possible sealing relationship in a position other than fully open, the methods according to said two French documents do not always provide a fully satisfactory valve arrangement.
From U.S. Pat. No. 4,702,156 it is known to use thermal moulding in order to produce a housing in a first step and movable parts for that housing in a second step in one and the same tool. This known method, however, relates to a through-flow channel with movable plates mounted therein for directing air flow, which movable parts steer the air flow in a desired direction but do not control the volume of flow. Similar methods are also described in GB-A-2 117 694 and FR-A-2 316 060.
There is a demand within the motor industry for a volumetric flow control valve for controlling the flow of air into an internal combustion engine. Accordingly, it is a general goal within the motor industry to reduce the number of parts, since this leads to lower production and assembly costs. Another general goal, which has been developed more recently, is to try to optimize the included components to improve comfort and energy consumption. This latter goal has, for instance, resulted in designs which permit a "smooth" driving style. One result of such a design is that butterfly plates have been provided with flow decreasing means in order to optimize the relation between the effective through-flow area as a function of the rotation of the butterfly and thereby counter-acting the undesired, non-linear relation (in certain cases almost a sinus function) which leads to a jerky manner of driving at small opening angles. Known butterflies have such flow decreasing means in the form of separate parts which protrude transversely (so called rucksacks) which have been fitted to the base- surface of the butterfly. Accordingly, an optimization in accordance with the latter object has in this case led to a non-desired result concerning the number of parts.
The need for accurately aligned and sealing components in butterfly valve arrangements is typified by the teachings of U.S. Pat. Nos. 4,740,347 and 3,771,764.